The first photo of light as both particle and wave

Since the days of Einstein, scientists have been trying to directly observe how light behaves both as a particle and a wave at the same time. Now, the first-ever snapshot of this dual nature has been captured, potentially opening up a new route towards quantum computing.

When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this ‘photoelectric’ effect by proposing that light – thought to only be a wave – is also a stream of particles.

But no experiment has ever been able to capture both of these ‘split personalities’ of light at the same time. The closest researchers of quantum mechanics have come is seeing either wave or particle, but always at different times.

Now, researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and Trinity College and the Lawrence Livermore National Laboratory in the US have done an experiment with a clever twist: using electrons to image light. The team is the first to capture a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle. The breakthrough work, spearheaded by Fabrizio Carbone at EPFL, was published today in the journal Nature Communications.

A new take on a classic effect

The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves travelling in opposite directions meet each other, they form a new wave that looks like it’s standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

The imaging was done at EPFL’s ultrafast energy-filtered transmission electron microscope – one of only two in the world. (Photo: EPFL.)

This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using an ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualise the standing wave, which acts as a fingerprint of the wave-nature of light.

While this phenomenon shows the wave-like nature of light, it also simultaneously demonstrates its particle aspect. As the electrons pass close to the standing wave of light, they ‘hit’ the light’s particles – the photons – thereby either accelerating or slowing down their speed. This change in speed appears as an exchange of energy ‘packets’ (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

New route towards quantum computing?

“This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: “Being able to image and control quantum phenomena at the nanometre scale like this opens up a new route towards quantum computing.”